Alterations and improvements

Before making any changes to your flat - whether "improvements" or changes in use, you need to check your Title Deeds and check if you need to get any consents or licences. There may well be legal consequences if you do not do this.

Internal changes and structural alterations

You are legally prohibited from interfering with any part that provides support and shelter to the building, or from interfering with the light to any flat. This means you cannot demolish walls without providing alternative structural support. You cannot leave openings in walls which would let damp into the building.

Apart from needing others owners' consent and building warrant, you also need to consider how such changes could affect your neighbours:

putting a kitchen or living room above or below a bedroom could be antisocial and lead to later problems.

replacing or renewing sound insulation between floors, particularly around new pipes.

using carpet rather than laminate flooring will greatly reduce noise problems

Legal reference

Windows

You may need planning consent to replace windows (check with your Council). Check also what your Title Deeds say.

Maintenance

You have a duty to maintain the parts of your building that provide support and shelter so you must keep your flat maintained. It is particularly important to keep windows and the mastic around them maintained and to deal with dripping overflows, both of which could lead to damp in flats below.